Alkaline anion exchange membrane (AAEM) fuel cells have received considerable interest as a high efficiency, low emission, and low cost energy converter. The use of non-precious metal catalysts in AAEM fuel cells provides a potential advantage over incumbent proton exchange membrane fuel cells. AAEMs may also find use in water electrolyzers for energy storage. Typical cationic functional groups for AAEMs include ammonium, sulfonium, phosphonium, pyridinium, imidazolium, benzimidazolium, and metal-cations, such as ruthenium. However, these generally degrade when exposed to solutions of high pH and at elevated temperature. Recently, an exceptionally hydroxide-stable polymer, mes-PDMBI-OH−, having the structure

was found to show no observable degradation in 6M KOH at room temperature or in 2M KOH at 60° C. Hydroxide-stability is conferred by introducing steric crowding around the C2-position of the benzimidazolium units. Mes-PDMBI-OH−, however, is water-soluble, requiring it to be blended to form water-insoluble membranes. Blending reduces the ion-exchange capacity (IEC) from 4.5 meq g−1 to 1-2 meq g−1, which limits the conductivity. Moreover, blends require the use of a high-boiling solvent, DMSO, for casting, which greatly limits processability, especially for catalyst layer fabrication.
Selective solubility of hydroxide-conducting polymers has previously proven elusive. Of the few ammonium-based ionomers developed, most are soluble in solvents such as NMP and DMSO, requiring higher temperatures for spray-coating and are more likely to contaminate the catalyst layer. For example, an ammonium-based poly(2,6-dimethyl-phenylene oxide) was described in Li et al., J. Am. Chem. Soc., 2013, 135, 10124-10133 to have solubility in methanol and ethanol solvents; but it appears to degrade to 90% of its original conductivity after 60 h in a fuel cell and the stability in its dissolved state was not described.
Thus, polymers which contain cationic groups in the backbone that are stable and soluble in polar solvents (e.g., alcohols) but insoluble in water are needed. The polymers can be suitable for ionomers in catalyst layers for fuel cells and electrolyzers. The present disclosure seeks to fulfill these needs and provides further advantages.